As it is known, rotary die cutting machines for cutting flat cardboard sheets and similar materials, have mainly two cylinders made of steel, where the cardboard sheet goes between them. One of the cylinders incorporates the die cutter, and it acts as the cutter, while the second cylinder acts as a base to let the blades of the cutter cut the cardboard sheet without being deteriorated. The dies on the die cutting cylinder pass through the work piece in different orientations to form products from flat sheet material such as corrugated sheets.
The second cylinder, which acts as a base, is provided with anvil cover, or a blanket of a relatively soft material, which is typically made of polyurethane, to let the blades cut the cardboard without deteriorating them.
More specifically anvil covers or blankets are thick bands designed to surround the cylinder, that is, to form a kind of cylindrical sleeve, which is sometimes fixed to the cylinder by dimensional interference between a key of the polyurethane anvil cover and the slot of the cylinder, by screwing or by other different means.
The critical point, and the place where the problems arise, is the key of the cover, due to the higher thickness of polyurethane in that zone regarding the rest of the cover. Due to this, the area of the key provides a “soft spot” that suffers a deformation during the die-cutting of the cardboard that is greater than the deformation suffered by the rest of the cover, which is directly supported by the steel cylinder. This fact is known as “spring effect”, and due to this great deformation of this “soft spot” a higher working pressure will be required, and the cardboard sheet could be deformed in the area of the key. FIG. 1 shows a section view of a blanket known in the prior art which schematically shows the “spring effect” on the “soft spot”.
Additionally, and regarding the joint of the ends of the bands or blankets wrapping the counter-dies forming a cylindrical sleeve, there are several different conventional systems used nowadays:
1. Straight joint: the joint of the band or blanket is straight, along the axial direction of the cylindrical sleeve, so it tends to open. This causes a bad cardboard processing when a blade affects this straight joint, producing a bad cutting, scoring, and other defects.
2. Wave joint: the joint of the band or blanket have sinuous curve geometry whereby the possibility of a blade incision in an area of opening is reduced. However, the absence of interference or friction as a dovetail, will present similar problems related to the opening as the straight joint option.
3. Jig-Saw joint: by means of the engagement of fingers, this type of joint reduces the separation or opening on the joint. However, it presents difficult both the assembly and disassembly which means higher production costs due to increased downtime of the machine that are necessary for replacement. Regarding the assembly, the jig-saw joint intrinsically causes the deformation of the section of the fingers, so it will be necessary to overcome major interference by engaging the fingers. With regard to the disassembly, the large angles which tend to have the dovetail of a jig-saw joint difficult to remove the cover due to the required deformation for let the fingers slide over them.
Cutting the covers to configure the closure zone has associated problems in the joint. It is caused by the deformation of the key are due to the accumulation of the material. These deformation problems do not appear when joint geometries are “open”, as disclosed in FIGS. 5a and 5b that is, with straight cuts, wave type, etc. (although these open geometries may have more problems around the joint area). However in case of “closed” geometries as dovetails, as disclosed in FIG. 5c, the large quantity of low hardness material at the key area causes that during the cutting exercise for the manufacture of the cover, the final geometry of the fingers do not correspond with those of the cutting blade as it forms pronounced curvatures in the section, as shown in FIG. 2.
This problem caused by the cutting in the key, to form the geometry of the finger, involves mounting complications because when attempting to introduce finger, in the hollows of which are previously fixed to the cylinder slot, large interference between these occurs. This is due to the deformation of the soft material (thicker in the area of the key) by expansion.
In order to try to solve these “spring effect” problems, there are some embodiments known in the prior art, as metal keys, or other key configurations as that shown in document U.S. Pat. No. 6,889,587B2. This document shows a configuration with plastic element with double hardness. In this case the key is divided into two substrates. The lower layer is part of an inner sleeve of greater hardness, vulcanized with the outer layer of lower hardness. However this layer does not reach the sides of the cover, in the axial direction of the shaft. On the other hand this layer occupies the full width of the slot. FIGS. 3 and 4 show an embodiment using a key made of hard material, which gives rise to openings in the joint, and poor fixing in the slot of the counter-die. Besides, since no deformation of the hard material is allowed, damage on the key and the slot of the counter-die occurs, and assembly/disassembly difficult increases.
Therefore the current joint systems for the blanket wrapping the counter-die undertake the processing of cardboard and difficult the assembly and/or disassembly thereof.